This invention relates to refrigerator appliances incorporating automatic defrost systems and more specifically to an improved sensing and diagnostic system for such appliances which detects malfunctions of the defrost system and alerts the user to the occurrence of the malfunction.
Automatic defrost systems for refrigerators are well known. Basically in normal operation a defrost cycle is periodically initiated by some form of timer. In the defrost cycle the normal refrigeration mode is interrupted and a defrost heater is energized to radiate heat in the vicinity of the evaporator for a sufficient period of time to melt the built-up frost on the evaporator. A defrost terminating thermostat responsive to the temperature of the evaporator opens when the sensed temperature indicates that frost has been removed.
In the event that the defrost system become inoperative, over a period of time frost will accumulate on the evaporator to such an extent that poor cooling performance of the refrigeration system results. Hence there is a need for a diagnostic system which can reliably detect such a failure and alert the user to the need for corrective action, before excessive frost has accumulated.
One arrangement for providing such a warning to the user is disclosed in commonly assigned U.S. Pat. No. 4,392,358 to Hicks. In Hicks the voltage across the defrost initiating switch is monitored. When operating normally a voltage is detected across this switch except during the timed defrost period and possibly for a short time thereafter until the defrost thermostat returns to its normally closed position. If, for example, the thermostat terminating switch is designed to stay open as long as 30 minutes, duration of a zero voltage condition under normal conditions would be not more than one hour. In Hicks the duration of the zero voltage condition is monitored and if found to exceed a predetermined time, which in Hicks was set at 3 hours, an alarm is energized signifying an open circuit failure of the defrost circuit.
This arrangement would detect an open circuit failure due to a break in the heating element or a malfunction of the thermostat switch causing it to remain open, but it would not detect a failure in the defrost timer. This deficiency is not of great importance in systems such as that disclosed in Hicks in which the defrost cycle is initiated at regular intervals under control of an electronic controller which performs the timing function since such controls are highly reliable. However, electro-mechanical timers are much more widely used in such appliances than electronic timers and this latter failure mode is one of the more common failure modes for systems employing an electro-mechanical defrost timer.
It would be desirable therefore to provide a fault detection arrangement for an automatic defrosting system in a refrigerator appliance which reliably detects a fault condition which prevents energization of the defrost heater regardless of the nature of the fault.
It is a further object of the present invention to provide an improved fault detection arrangement for an automatic defrosting system in a refrigerator which monitors operation of the defrost circuit and which alerts the user to a fault condition if the defrost heater is not energized within a predetermined time period regardless of the nature of the fault condition preventing such energization.